The answer to your question would be to apply the Cobra 148GTL instructions to your radio.
But, if that is an older finer Uniden.... Don't do yourself the disservice of making it a lesser radio by modifying it. I would leave it alone and purchase an amplifier for it. A receiving station most likely won't hear the difference between the Mosfet modified radio and the stock radio anyway.
Others opinions may differ, but I think it is difficult to make that particular radio "better". It is pretty easy to make it worse however.
Mopar